The present invention is directed to a method and apparatus for electron beam irradiation of a single layer or multi-layer article, and resulting products. More particularly, the invention is directed to use of a low loss electron beam path to irradiate an electron beam modifiable material coated on an electron beam degradable substrate.
In recent years, electron beam radiation has increasingly been used for modifying various materials, including polymerizing, crosslinking, grafting, and curing materials. For example, electron beam processing has been used to polymerize and/or crosslink various pressure-sensitive adhesive formulations coated on film substrates, to graft coatings onto substrates, and to cure various liquid coatings, such as printing inks. Using an electron beam to modify a material avoids the need for coating solutions, including those comprising volatile organic compounds (xe2x80x9cVOCsxe2x80x9d). This allows for a reduction in VOC emissions, and a concurrent reduction in energy costs and environmental or occupational hazards.
Unlike ultraviolet (xe2x80x9cUVxe2x80x9d) radiation, which is also used to crosslink, polymerize, graft, and cure various materials, electron beam radiation does not require the use of an initiator. In addition, electron beam radiation is readily absorbed by all organic materials, even those materials that are not readily modified by UV radiation, such as thick, opaque materials and those that resist UV modification, such as allylic, olefinic, and unsaturated compounds. Polyethylene is an exemplary unsaturated compound that cannot readily be cured by UV radiation, but is curable by electron beam radiation.
Although electron beam radiation has many advantages, it does have some limitations. These limitations include the fact that electron beam generating equipment has traditionally been relatively expensive. The high expense is at least partially associated with the need for large power supplies, lead shielding, high voltage components, and safety monitoring equipment. In recent years, manufacturers have been able to build less expensive, more compact, lighter electron beam equipment by lowering the voltage of the electron beam to 125 kilovolts (kV) or less. For example, Energy Sciences, Inc. of Wilmington, Mass.; Advanced Electron Beam Technologies, Wilmington, Mass.; and American International Technologies, Inc. of Torrance, Calif. are manufacturers of compact, low cost electron beam generators. These machines make it possible to lower the purchase and operating costs of electron beam radiation equipment.
Another significant limitation of electron beam radiation is that electrons frequently penetrate too deeply into the material being irradiated. High voltages are frequently used to obtain a reasonably uniform dose over the entire cross-section of an electron beam modifiable coating, but this can result in a significant amount of energetic electrons passing into layers below the electron beam modifiable coating. This becomes a problem in multi-layer materials that comprise a coating of material that is being modified, and a substrate or backing of material that can be damaged by electron beam radiation. Paper, polyvinyl chloride, polypropylene, and TEFLON are all materials that often are used as substrates for adhesives, yet are susceptible to degradation from electron beam radiation. Electron beam radiation can cause the substrate to become brittle or otherwise degraded. The result is a deteriorated substrate that makes the product either lower quality or unusable for its desired application.
Existing electron beam generation systems do not adequately address the problems of high machine costs and satisfactorily modifying a coating without degrading the substrate. Consequently, a need exists to control electron beam irradiation such that the electron beam penetration is substantially limited to specific layers of the irradiated material, preferably just the electron beam modifiable coating of the material.
The present invention is directed to an apparatus and method for delivering electron beam radiation to a material, particularly a multi-layer material having an electron beam modifiable coating and an electron beam degradable substrate. The invention is also directed to products manufactured using the apparatus and method of the invention. At least one embodiment of the present invention allows one to control the dose (energy deposited per unit mass) delivered to particular depths in an irradiated material.
One aspect of the invention is directed to an electron beam apparatus comprising an electron beam source, a window proximate the electron beam source comprising a polymeric film having at least two surfaces, a protective layer resistant to free radical degradation on at least one surface of the polymeric window, a support proximate the window on which to place materials to be irradiated by the source, and a gap between the window and support.
Another aspect of the invention is directed to a window for use with an electron beam source comprising a polymeric film having at least two surfaces, the film having a protective layer resistant to free radical degradation on at least one surface wherein the film is able to contain an environment having a pressure of less than 10xe2x88x924 Torr.
Another aspect of the invention is directed to a method of irradiating an article with an electron beam comprising providing an electron beam source; providing a window for use with the electron beam source, the window comprising a polymeric film having at least two surfaces a protective layer resistant to free radical degradation on at least one surface; and irradiating the article through the window with electrons from the electron beam source.
Another aspect of the invention is directed to a method of modifying the properties of an article having two or more layers comprising providing an article having an electron beam modifiable first layer and an electron beam degradable second layer proximate the first layer; providing an electron beam source for which energy, voltage, and current levels may be adjusted; providing a window between the electron beam source and the article to be irradiated, wherein a gap exists between the window and article, the window having a unit path length of 3 to 50 grams per square meter, setting the electron beam source energy to between 50 and 150 keV; adjusting the electron beam source voltage and current, and adjusting the gap distance between the window and article such that the electron beam can modify the first layer without substantially degrading the second layer, and irradiating the article with an electron beam from the electron beam source.
Another aspect of the invention provides an electron beam modified article comprising an electron beam degradable backing material, and an electron beam modified coating on the backing material, the 30 micrometers of the electron beam degradable backing adjacent the modified coating having absorbed between 0.1 and 40 mJ/cm2 of energy.
Another aspect of the invention provides an electron beam modified article comprising an electron beam degradable backing material, and an electron beam modified coating on the backing material, the modified coating being free of release material contamination. Because the present invention allows an electron beam modifiable layer to be modified, e.g., cured, directly on an electron beam degradable backing without materially degrading the backing, the modifiable layer is not required to be modified on a release material, such as silicone, then transferred to the backing. This eliminated the possibility of the modifiable layer being contaminated with release material.
In irradiating an electron beam modifiable material coated on an electron beam degradable substrate, it is important to provide a dose to, and through, the irradiated material that will adequately modify the modifiable layer so it will be useful for its intended purpose and so it will adhere to the substrate. However, it is important that the dose is not excessive. For example, when an adhesive layer on a substrate is irradiated, the surface dose must be sufficient to impart important adhesive properties such as cohesive and adhesive strength, but the dose should not be so high that it over-modifies, e.g., over-crosslinks, or degrades the adhesive layer (which would limit its adhesive properties). The dose must also be sufficient to modify the adhesive at the adhesive/substrate interface so the adhesive will bond with the substrate. However, the interface dose should not be so high that the substrate is significantly degraded.
The electron beam apparatus of the present invention includes an electron beam source configured and arranged to direct electrons into a material, most suitably a multi-layer material having both an electron beam modifiable upper layer and a electron beam degradable lower layer. In traveling from the electron beam source, the electrons pass from a vacuum environment through a window foil having low electron absorbency properties (a xe2x80x9clow lossxe2x80x9d window) into an atmospheric pressure environment containing the material to be irradiated. The route of the electron beam from its source, through the low loss window, to the irradiated material is sometimes referred to herein as the low loss path. By using a low absorbency window, even a relatively low voltage electron beam can pass through the window with only a slight reduction in power. The resulting electron beam is able to enter and modify the coating of the irradiated material, preferably without entering and degrading any substrate.
Appropriate window materials for use in the low loss path include polymeric films, such as polyimide films. A protective layer is placed on at least the window surface facing the atmospheric pressure environment to reduce free radical degradation and thus improve performance and durability. The protective layer may be a thin layer of aluminum or other metal that protects against free-radical degradation. Preferably it also enhances electrical and thermal conduction along the film.
After the electrons pass through the window, they travel through a gap between the window and the material being irradiated. The gap normally contains nitrogen gas or another inert material maintained at approximately atmospheric pressure. The gap distance is preferably minimized to increase the dosage of electron beam radiation delivered to the modifiable coating and to reduce the dosage absorbed by electrons in the gap. Reducing the gap distance also improves the energy efficiency of the apparatus such that lower voltages may be used to irradiate a material. The gap between the window and the irradiated material is between about 2 and 100 millimeters in certain embodiments, between 4 and 50 millimeters in other embodiments, and between about 5 and 20 millimeters in yet other embodiments. The preferable gap size will depend on factors such as the window material, the presence of a window clamp structure, the voltage used, and the thickness of the modifiable layer.
The amount of electron energy absorbed by the window, gap, coating layer, and any substrate layer as the electron beam travels through these regions can be determined and plotted on a depth/dose curve, which plots dose absorbed against distance from the electron beam source. The dimensions of the curve may vary depending on numerous conditions, but it will typically have a peak where energy absorption is the greatest. In conventional electron beam systems, this peak often exists in the window or gap region. The ideal depth/dose curve would have a square wave shape such that the window and gap absorbed no energy, the modifiable material layer absorbs a uniform amount of energy through its total depth and the degradable substrate absorbed no energy.
A principle advantage of the low loss beam path is that it can shift the absorption peak, also referred to in the art as xe2x80x9cback scatterxe2x80x9d peak, of the depth/dose curve out of the window/gap region and into the coating layer region such that the depth/dose curve better approximates the ideal square wave curve. At the same time, the lower voltage permitted by the low loss beam path characteristically produces a depth/dose curve having a steep negative slope over the remaining depth of penetration subsequent to the absorption peak. Accordingly, appropriate selection of window materials and gap distances allows the generation of a depth/dose curve having a declining slope that may closely coincide with the interface between the substrate and the coating.
Per the present invention, the electron beam radiation dosage may rapidly diminish upon entry into the irradiated material such that the dose received by a coating may be significantly more than that received by a substrate. The proportion of the total dose received by the substrate is affected by factors such as the shape of the depth/dose curve, the window material, the gap distance, the voltage required to achieve satisfactory modification of the coating, and the thickness of the substrate. In some embodiments the dose may be 1 to 5 times greater at the coating surface than at the coating/substrate interface. The acceptable surface to interface dose ratio will largely depend on the amount of radiation the coating layer can receive without becoming degraded or over-modified, e.g., over-crosslinked.
Conventional electron beam paths, e.g., those with a 12 micrometer titanium window, operating at voltages above approximately 150 kV, generally produce relatively flat, wide depth/dose curves. When a high surface dose is used, the substrate may suffer a substantial amount of degradation because the interface dose and total dose to the substrate will typically increase as the surface dose increases. The inventors have found, surprisingly, that a low loss beam path can have a relatively high but narrow depth/dose curve such that a high surface dose does not necessarily result in a high interface dose. Accordingly, an electron beam modifiable layer, such as an adhesive, can be successfully modified with an electron beam dose that is as much as 5 times greater at the coating surface than at the coating/substrate interface. Because of the shape and placement of the depth/dose curve produced with a low loss path, a sufficient dose can be provided to the adhesive layer, and the interface can be sufficiently modified to adhere to the adjacent substrate, with minimal electron beam penetration into the substrate.
To improve upon the predictability of the dose of electron beam radiation at varying depths in the irradiated material, a Monte Carlo code can be used to predict depth and dose values based upon the window material and the gap distance. These predictions facilitate adjustment of the electron beam dose at various depths in the irradiated material, and allow for optimal dosage delivery and modification of a coating without damage to the substrate. The electron beam radiation used to irradiate the coated substrate preferably operates at a voltage of about 30 to 150 kV, more preferably about 50 to 100 kV, and most preferably about 50 to 75 kV. Selection of voltage can determine the shape of the depth/dose profile (and therefore the ratio of surface to interface doses). Selection of current can determine the actual dose delivered to the irradiated material. Adjusting the current can, for example, change the interface dose.
The invention is further directed to a product, specifically an electron beam modified article. A product may comprise one or more electron beam modifiable layers. In some embodiments, the article comprises one or more electron beam modified coating layer(s) on an electron beam degradable substrate. The invention includes embodiments wherein an electron beam degradable substrate shows acceptable, minimal, or no electron beam degradation after being irradiated. The targeted interface dose is one that would produce minimal degradation while allowing the coating to adhere to the substrate such that a viable tape product is prepared.
The above summary is not intended to describe every embodiment of the present invention. Other aspects and advantages of the invention will become apparent upon reading the following description of drawings and detailed description.